This invention relates to apparatus for developing a latent electrostatic image wherein a magnetic brush development system develops a latent electrostatic image supported on an image retaining member to make it visible, and more specifically, this invention relates to a magnetic development apparatus in which improved control is afforded over the developer materials during the imaging process.
For example, in the art of xerography, a xerographic plate, usually comprising a photoconductive surface placed over a conductive backing, is first charged uniformly and then exposed to a light image of an original to be reproduced. Under the influence of the light image, the photoconductive surface is discharged to form what is known as a "latent electrostatic image". Conventionally, the latent image is developed by contacting the charged image areas with an oppositely charged toner material which has been specifically developed for this purpose. The oppositely charged toner particles are electrostatically attracted into the charged image areas thus making the latent image visible.
In theory, areas of greater charged density should be developed as areas of high toner density while areas of lesser charge concentration should be proportionally less dense. However, in practice, this has been found to be difficult. Large "solid" areas of charge concentration supported on a surface, such as a photoconductive plate, typically exhibit a non-uniformity of development when contacted with a toner material. It is believed that the flux density of the electrostatic force field associated with the solid areas varies with the stronger forces located along the fringes or edges of the images. The edge areas, therefore, develop at a faster rate than the interior areas although both are similarly charged. Because of the phenomena, solid areas which must be developed within a relatively short time period, as in automatic xerographic machines, often appear washed out or underdeveloped.
In multiple roller magnetic brush development systems there are several types of "image history" effects that can be caused by the developer traveling through the development zone in a straight line. One type of effect is where toner is scavenged or reclaimed by undertoned developer that overtakes a medium density image area after having previously given up a substantial part of its toner to develop a dense image area circumferentially ahead of the medium image.
Various inventions have been advanced with the idea being to enhance solid area development, among those being U.S. Pat. No. 3,638,611 which discloses an increase in uniformity of development by the use of a biased development electrode with pins attached that is capable of being moved in a lateral direction substantially perpendicular to the developer flow. In a patent to Donalies et al., U.S. Pat. No. 3,331,355, a development apparatus is shown in which donor loading is effected by directing developer into direct and electrical contact with a biased electrode by dropping a developer mix through an electrode having an array of biased wires on a grid supported over the donor member. Other patents that relate to enhancing solid area development and uniformity in a different manner include U.S. Pat. Nos. 2,846,333; 3,147,472; 3,336,905; 3,380,437; and 3,558,339. International Business Machine Technical Disclosure Bulletin in Vol. 2, No. 2, August 1959, pages 4 and 5 also relates to development of electrostatic images all of which are hereby incorporated by reference.
It is, therefore, an object of this invention to improve xerographic development and overcome the above-noted deficiencies.
A further object of this invention is to reduce image history defects and enhance solid area development.
Yet a further object of this invention is to improve automatic xerographic development by providing a developing apparatus that provides a control over the developer material whereby image areas of varying sizes and densities are developed uniformally.
These and other objects of the present invention are obtained by inserting a comb or weir-like oscillating structure between successive magnetic brush rollers in the region where developer is transferred from one roller to the next, developer that has been used to develop at one position on one roller can be directed to a new position on the next. Scattering developer between development steps will tend to eliminate or reduce any non-homogeneities in the toner distribution created by an earlier development process.
For a better understanding of the invention as well as other objects and further features thereof, reference is had to the following detailed description of embodiments of the invention to be read in connection with drawings wherein: